This application claims the benefit of Korean Patent Application No. 1999-29057, filed on Jul. 19, 1999, under 35 U.S.C. xc2xa7119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a digitizer, and more particularly, to a digitizer for use in a liquid crystal display (LCD) device.
2. Discussion of the Related Art
A digitizer is one of a class of devices that responds to location information input by a finger or a stylus on a touch panel to generate paired analog coordinate signals corresponding to the location on a touch panel to which the finger or stylus has been applied. In a digitizer, the generated analog coordinate signals are digitized and conveyed to a display device for use in a.notebook computer. Digitizers have also been used generally in computer graphics, computer aided design (CAD), and computer aided manufacturing systems.
FIG. 1 shows a resistive touch panel 10 of a conventional digitizer disclosed in U.S. Pat. No. 4,853,493, having a resistive layer represented at grid xe2x80x9cRgxe2x80x9d, a plurality of linear resistors RLXN and RLYN, chain resistors RCHX and RCHY, and corner resistors RCXN and RCYN. Corner resistors RCX1 and RCY1 are coupled or connected with terminal V1; corner resistors RCX2 and RCY2 are coupled or connected with terminal V2; comer resistor RCX3 and RCY3 are coupled or connected with terminal V3; and comer resistors RCX4 and RCY4 are coupled or connected with terminal V4. The terminals V1, V2, V3 and V4 are coupled or connected to a digitizer drive circuit (not shown).
To derive signals representing analog coordinate pairs corresponding to the position of the stylus or the finger in the conventional digitizer of FIG. 1, a reference potential is alternately applied to every two adjacent terminals, moving in a clockwise or counterclockwise direction using predetermined switching techniques. The reference potential is derived from a voltage source (not shown). For example, in case of a clockwise direction, when an a.c. reference voltage of 5 volts is applied to the terminals V1 and V2, the terminals V3 and V4 are retained in a xe2x80x98groundxe2x80x99 condition. Further, when an a.c. reference voltage of 5 volts is applied to the terminals V2 and V4, the terminals V1 and V3 are retained in a xe2x80x98groundxe2x80x99 condition. Electrical signals from the digitizer drive circuit are applied to the chain resistors RCHX and RCHY through the corner resistors RCX and RCY and then applied to the grid resistor Rg through a plurality of the linear resistors RLXN and RLYN. The resistance of the linear resistors RLXN and RLYN differs according to their locations so as to form equipotential lines perpendicular to the applied signals. In other words, in order to form the equipotential lines, the linear resistors RLXN and RLYN become shorter in length or wider in width as they get further from the corner resistors RCXN and RCYN, respectively. This is because the voltage drop between the terminal and the grid resistor Rg becomes greater as the linear resistors RLXN and RLYN get farther from the corner resistors RCXN and RCYN.
However, as shown in FIG. 2, the potential distribution is not uniform, and the potential distribution becomes more uniform closer to the central portion of the grid resistor Rg. This is because the arrangement of the linear resistors RLXN and RLYN is not as dense as the arrangement of the grid resistor Rg. Therefore, there are connection portions and non-connection portions between the borders of the grid resistor Rg and the linear resistors RLXN and RLYN, resulting in a potential difference between the confection portions and the non-connection portions. Therefore, portions xe2x80x9cBxe2x80x9d of the grid resistor Rg, in which a significant potential difference occurs, cannot serve as an active region xe2x80x9cCxe2x80x9d for sensing a location of a stylus or a finger. Thus, the non-active area, which is unusable for sensing, is relatively large.
FIG. 3 shows a touch panel 12 of a digitizer disclosed in U.S. Pat. No. 4,755,634. As shown in FIG. 3, a resistive touch panel 12 includes a plurality of parallel resistive elements RSTR in the form of a strip, whose ends are interconnected with two chain resistor elements RCH through linear resistor elements RLn. The linear resistors RLn differ in resistance so that the same potential is applied to each strip resistor RSTR. The chain resistor elements RCH are connected to the terminals V1, V2, V3 and V4 through input resistor electrodes RIN1, RIN2 RIN3 and RIN4, respectively. The terminals V1, V2, V3 and V4 are connected to the digitizer drive circuit (not shown).
To derive signals representing coordinate pairs corresponding to the position of a stylus or a finger using the touch panel of FIG. 3, four electrical signals are simultaneously applied to the terminals V1, V2, V3 and V4 from the digitizer drive circuit (not shown). To detect precise x-y coordinate values, it is preferred that the parallel resistive elements RSTR have equipotential lines at the same locations along the y-axis of each parallel resistive element RSTR, regardless of the difference in the signals respectively applied to the terminals V1, V2, V3 and V4.
However, as shown in FIG. 4, the resistive touch panel 12 does not show such desired equipotential distributions when different electrical signals are applied to the terminals V1, V2, V3 and V4. For example, different potentials at each terminal can result when the wires that connect each terminal with the digitizer drive circuit differ in resistance, so that the electrical signals applied at each of the terminals differ. Thus, the actual potential distribution throughout the panel is not uniform and differs from the designed potential distribution. This non-uniform potential distribution results from the resistor structure of the resistive touch panel 12. Because the parallel resistor elements RCH of the resistive touch panel 12 are arranged only in the direction of the x-axis, the potential distribution of the parallel resistor elements RCH shown in FIG. 4 is inferior to that of the grid having an x-y coordinate arrangement. That is, the potential lines are skewed, and therefore, the digitizer cannot produce or detect precise location information indicative of the location of a stylus or a finger.
Accordingly, the present invention is directed to a digitizer that substantially obviates one or more problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a digitizer having an improved potential distribution.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof, as well as the appended drawings.
To achieve the above object, and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a touch panel for sensing a point of contact on a display. The present invention also provides an apparatus for locating a selected touch point.
One embodiment of the present invention provides an apparatus for locating a selected touch point physically touched by a human finger or a stylus and producing an electrical signal corresponding thereto. The apparatus of this embodiment includes an insulating substrate and a resistor touch pattern supported on the insulating substrate. The resistive touch pattern includes a plurality of strip resistors that are parallel to a center line. The center line is defined by the plurality of strip resistors. Each of the strip resistors has two ends. The resistive touch pattern also includes a plurality of linear resistors that have first and second ends. Each of the first ends of the linear resistors is connected respectively to one of the two ends of the strip resistors. The resistances of two linear resistors connected to the same resistor are equal. The resistance of the linear resistors decreases with their distance from the center line. The distance between the first and second ends of each linear resistance is equal. The resistive touch pattern of this embodiment also includes first and second chain resistors connected to each of the second ends of the plurality of the linear resistors. Each of the chain resistors has a first portion perpendicular to the strip resistors, and second and third portions extending from both ends of the first portion in a direction away from and parallel to the strip resistors. The resistive touch panel of this embodiment also includes first and second edge resistors that are parallel to each other and connect to corresponding second and third portions of the chain resistors. Each of the edge resistors in this embodiment has the same length as the strip resistors. The resistive touch panel of this embodiment also includes first and third terminals, respectively, located on an intermediate point of each first portion of the first and second chain resistors. Each of the first and third terminals is connected to a drive circuit. The first and third terminals are driven simultaneously. The resistive touch panel of this embodiment also includes second and fourth terminals, respectively, located on an intermediate point of each of the edge resistors. Each of the second and fourth terminals is connected to the drive circuit. The second and fourth terminals are driven simultaneously. The apparatus of this embodiment further includes a protective insulating plate arranged over the resistive touch pattern. The first and third terminals and the second and fourth terminals are driven by the drive circuit alternatively. The apparatus further includes a transparent conductive plate arranged between the substrate and the protective insulating plate, and dot spacers located between the resistive pattern and the transparent conductive plate. The transparent conductive plate has a low-resistive metal film surrounding the edge of the transparent conductive plate and a lead line for outputting the electrical signal representing a location of the stylus or the finger.
Another embodiment of the present invention includes a touch panel having a plurality of strip resistors parallel to one another and first, second, third and fourth electric signal terminals. The first and third terminals are positioned on the midpoint of opposite edges of the touch panel. The second and fourth terminals are positioned on the midpoint of opposite edges of the touch panel. This embodiment further includes respective linear resistors connected between a respective end of a corresponding strip resistor and one of the electric signal terminals. Each linear resistor has a resistive value matched to the corresponding strip resistor to produce an equal potential at a corresponding selected point on all of the strip resistors.
Another embodiment of the present invention provides a touch panel for sensing a point contacted on a display that includes a plurality of strip resistors parallel to one another. Each of the strip resistors has first and second ends. The touch panel of this embodiment further includes a plurality of electric signal terminals. The first and third electric signal terminals are located, respectively, on first and third edge portions of the touch panel. The first and third edge portions are opposite to one another. The second and fourth electric signal terminals are located, respectively, on second and fourth edge portions of the touch panel. The second and fourth edge portions are opposite to one another. Each of said electric signal terminals is located at the midpoint of its respective edge portion. The touch panel further includes a plurality of linear resistors. For each strip resistor, a first respective linear resistor is connected between the first end of the corresponding strip resistor and one of the electric signal terminals. A second respective linear resistor is connected between the second end of the corresponding strip resistor and another of the electric signal terminals. Each of the linear resistors has a resistive value selected for its corresponding strip resistor to produce an equal potential at a corresponding selected point on all of the strip resistors.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.